In the past, procedures for producing field effect transistors have utilized semiconductor material such as silicon or germanium that is not effected by photolithographic etching techniques. These same procedural steps for making silicon or germanium semiconductor transistor devices were attempted on thin film field effect transistors utilizing polycrystalline cadmium selenide, polycrystalline cadmium sulfide or other semiconducting compounds or mixtures that are effected by acid etching. Due to the semiconductor material being effected by acid etching, transistors with these materials could not be produced to acceptable standards. More specifically in relation to the prior art, the field effect transistor as manufactured utilizing prior art uses an extremely simple, low cost process by which all of the working elements of the transistors are deposited by conventional vacuum deposition methods, by depositing the various materials of the several layers required in the position, sequence and patterns required to form the finished transistor. These working elements are deposited, one layer at a time on a flat insulating substrate at a series of working stations. The patterns deposited are defined by a series of aperture masks. These masks are thin, flat metal sheets, in function similar to stencils, having the required pattern as aperture etched through the stencil or mask. At each work station, the selected mask is placed in its correct position against the substrate, after which the required material is deposited on the substrate in the correct thickness required and the pattern as determined by the aperture pattern in the mask. The number of stations and their sequence are organized to produce working arrays of thin film field effect transistors by the described deposition of the various elemental patterns comprising the working thin film field effect transistor and its associated patterns during a single trip through the vacuum chamber. This simple, easily accomplished method for manufacturing thin film transistors has led prior investigators to concentrate their efforts on improving the process, without, as must be concluded from the available evidence, being aware of the limitations thereof. These limitations are imposed by the etched patterns in the aperture masks used to define the various elements of the transistors. For mechanical reasons the shadow masks are made of sheet metal one mil in thickness, with etched patterns. This manufacturing process is limited in practice to producing masks with apertures no smaller than approximately two mils in the narrowest dimension.
The result is that this opening width determines the narrowest width of transistor element that can be deposited through the openings of such masks. Moreover, the mask manufacturing process leaves irregular edges on the mask apertures, as a result of varying rates of etch attack on the randomly oriented crystallities of the mask metal microstructure. These factors combine to limit the performance of prior art thin film transistors made by successive deposits as defined by such masks.
It is well known to those skilled in the art that there are several critical dimensions that control the performance of thin film field effect transistors. It is obvious that the current rating of the transistor is dependent upon the cross sectional area of the semiconductor. That is, the thickness of semiconductor, which is readily deposited to the precision required, times the width of the semiconductor deposit, which is difficult to define by the imperfect mask. Even more critical dimensional requirements are found in other parts of the transistor, to the extent that many narrow structures, such as gate dimensions in the desirable three to five micron source-to-drain length cannot be produced by the aperture masks used in manufacture of thin film field effect transistors.
In conventional silicon based microelectronics, such dimensional accuracy is obtained by etching the transistor, by which the patterns in the required dimensional accuracy and small sizes are readily produced by photolithographic techniques. These techniques are universal in the manufacture of silicon and germanium devices, and are so well understood as to be common knowledge among those familiar with the art. As mentioned before, state-of-the-art in manufacturing thin film field effect transistors in their manufacture by vacuum deposition of their various elements in their finished configuration by the use of a sequence of aperture masks without lithographic techniques being applied. The few attempts to use photolithographic definition of etch pattern have failed in that prior efforts have attempted to use photolithography on unprotected (uncoated) semiconductor layers, and contact the unprotected surface of the semiconductor film with reactive chemicals at some stage of the photolithographic process. This contact on the very thin layer of semiconductor material has invariably destroyed transistor action, either by dissolving or contaminating the semiconductor. The net effect is that "Prior Art" has established the belief that the photolithographic techniques, so universal in manufacturing semiconductor devices from silicon or germanium, cannot be effectively utilized in the manufacture of thin film field effect transistors. Therefore, it can be clearly realized that there is a need for a method by which thin film field effect transistors can be produced accurately and with small percentage variation in properties utilizing photolithographic techniques.
Therefore, it is an object of this invention to provide a method by which the photolithographic technology from semiconductor manufacture can be successfully applied to make thin film field effect transistors according to this invention.
Another object of this invention is to provide a method for construction of thin film field effect transistors with configurations suited to utilization of photolithographic processing as a means of obtaining very accurately defined small dimensions that are required for transistors of superior characteristics.
A further object of this invention is to provide a method by which photolithographic processes can be used and in which the semiconductor material is protected from coming in contact with acid etching in the photolithographic process.
Still another object of this invention is to provide a simple method for making thin film field effect transistors by utilizing known techniques for each of the individual steps.
Other objects and advantages of this invention will be obvious to those skilled in this art.